1. The Field of the Invention
The present invention is concerned with apparatus for monitoring the carbon content of boiler flue ash.
In coal fired power stations it is normal to use pulverised fuel to ensure efficient combustion. Nevertheless some of the the combustible material in the fuel may not be burnt. The ash resulting in such power stations comprises very fine particulate material, some of which is entrained in the combustion gases flowing through the boiler flue. Such ash is typically removed from the flue gases discharged by the power station by filters and electrostatic separators.
It is useful to be able to determine the efficiency of combustion in a power station boiler and one way of doing this is to determine the level of carbon remaining in such boiler flue ash.
2. Prior Art
Reference should be made to Laboratory Note No. Rd/L/N 101/77 of Central Electricity Research Laboratories, Leatherhead, United Kingdom. This Note is entitled "C.E.R.L. On-Line Carbon-in-Ash Monitor", by J. E. Roughton and W. L. Snowsill, and is dated 8th Aug. 1977. The Note has been available since September 1977 to interested members of the public on request from the Central Library of the Central Electricity Generating Board, Sudbury House, 14 Newgate Street, London, EC1A 7 AU.
The above referred Laboratory Note discloses a carbon-in-ash monitor employing a fluidised bed furnace to burn any carbon in sampled boiler flue ash and evolve carbon dioxide therefrom. Fluidising gas is provided to the fluidised bed at a measured flow rate and a batching arrangement feeds successive batches of ash of measured mass to the furnace at a measured frequency. The amount of CO.sub.2 evolved is then monitored to determine the carbon content of the ash. The apparatus operates with successive batches of ash delivered to the furnace so that the level of carbon in the ash flowing in the boiler flue can be monitored substantially continuously. The total response time of the apparatus, from drawing a sample of ash from the flue gas, to providing an indication of the carbon content of that ash, is from one to two minutes. Successive batches of ash can be delivered to the fluidised bed at typically 30 second intervals. The time taken for a particular batch of ash to be completely burned in the fluidised bed is less than 30 seconds.
Conveniently batches of constant mass are delivered at a constant rate and the flow rate of fluidising gas, typically air, through the fluidised bed is also maintained constant, so that analysis of the carbon dioxide concentration in the gas leaving the fluidised bed provides a direct measure of the carbon content of the ash.
The batches are provided using a vibratory table to transport ash from a point of supply from the flue sampler, so as to be delivered in a stream from the table to a batch collector.
In the described arrangement, the batch collector is an over-centre stable balance arm with a collection bucket at one end located to collect ash from the stream delivered from the vibratory table. When the amount of ash collected in the bucket exceeds a preset mass, the arm over balances, taking the bucket out of the ash stream and depositing the batch in a collection funnel. Remaining ash on the vibratory table continues to flow in the stream from the table to waste.
It has proved difficult with this arrangement to provide accurately controlled batch sizes. Further the waster ash from the vibratory table tends to foul the balance mechanism.